The most destructive structural failure of Bangladesh is Rana Plaza, Savar, Bangladesh. More than 1000 people were killed and another 2400 are rescued in badly injured condition. Bangladeshi people and whole world as well want to know causes of failure of this building. Here in this post we will discuss about the possible causes of failure of Rana Plaza.
We all know that the structure was built with poor quality materials and there may have weakness in foundation. The structure was underdesigned, no doubt, as the structure was primarily designed and constructed as six storied building and disobeying law and not revising the design or retrofitting the existing underdesigned building, upper four stories were built. Top floor was in under construction during failure. ADPC and Department of Disaster and Management of Dhaka University jointly expressed their opinion that the columns were underdesigned (the visible collapsed elements).
They have concluded that the column size was found 10”X12” up to first four stories and the size was reduced to 7”X8” in the next phase of construction. Such a under size columns are not even imagined in residential buildings rather it was a commercial cum factory building having thousands of sewing machine and generator in upper factory floor.
The head of inquiry team, formed by Bangladesh government, Main uddin Khandakar, told the vibration of generator together with thousands of sewing machine led collapse of the structure. We will learn about some factors about frequency of machines and structures structure itself in the next paragraph.
Regarding machine, the frequency depends on type of machine and can easily be found from the technical manual of respective machine. Normally we used to provide machine above ground and necessary isolation is made to avoid impact of machine on structure. It is unusual to provide machine in upper floor unless condition forced to do that. If they are provided in upper floor, the usual procedure is to keep a certain difference in frequency of both (machine and structure). It is usual to keep the difference (3-5) Hz to avoid resonance.
Frequency of structure can conveniently determined by H/46, where H is height of building. Other methods are also applicable. In structural software like STAAD.Pro we can determine Rayliegh frequency of structure easily. In calculating frequency of machine the soil below the foundation, including foundation that can be influenced by machine should also considered in calculating frequency.
Now consider Rana Plaza, their four generator were serving with numerous sewing machines in upper floor. Was that designed for such vibration? In Bangladesh, many building designed or are designing without knowing purposes of building to be served for or after construction the occupancy criteria is changing. Many owners are still changing plan and elevation without consulting with designer or informing the authority. So why should these structures not collapse like Rana Plaza?